While phenol-formaldehyde resins have long been used for the preparation of wood composite for exterior use, urea-formaldehyde (UF) resins have been preferred for use in the preparation of wood composites, particularly wood composites for interior use, such as particleboard, medium density fiberboard and other composites made from small pieces of wood. UF resins have been a binder of choice because of their processing advantages and low cost relative to other typical wood adhesives. UF resin-based adhesives have good bonding properties and other characteristics that permit them to be used in high-speed processes for the preparation of the various boards or wood composite products. In the manufacture of composite board products, short press cycles can be achieved with urea-formaldehyde resin-based adhesives. Also, urea-formaldehyde adhesives have a desirable level of “tack”, causing adhesive-treated particles to stick to each other, so that mats made from a “tacky” finish tend to be self-sustaining in shape, which facilitates handling during board manufacture. UF resins also cure with a light colored glueline, which is advantageous in decorative panels.
Urea-formaldehyde resins are typically prepared by reacting urea and formaldehyde at a suitable mole ratio to form various methylolated ureas and their higher condensation products. The composition of any particular resin depends, among other factors, on conditions such as temperature, pH and time for the reaction.
Wood composites made with an adhesive binder containing a urea-formaldehyde resin have generally been limited to applications where exterior durability is not required. There is further a growing demand in the marketplace for materials derived in whole or in part from sustainable resources (“Bio-Based” materials), as well as materials that are compliant with environmental standards such as the LEED standards (Leadership in Energy and Environmental Design) as put forth by the USGBC (U.S Green Building Council).
Lower formaldehyde-emitting wood composite products and methods of producing them are thus desirable for manufacturers currently producing UF resin-based adhesive binders. One approach to obtaining lower formaldehyde products has been to use resin compositions with lower F/U molar ratios in the adhesive binders. Such resin compositions tend to be slower curing than the higher mole ratio, more reactive resins. Reducing the mole ratio of formaldehyde in such resins can therefore result in compromised board properties, such as decreased internal bond strengths, due to a lower extent of cure under equivalent pressing conditions. Compromised board properties such as decreased internal bond strengths can lead to delamination.
For at least these reasons, resins and additives that can improve board properties (especially at short press times), while maintaining equivalent, or even lower formaldehyde emissions, are desired. There also remains a need a need in the art for wood products which use an adhesive binder having a cure rate that is comparable to that of urea-formaldehyde, and which has improved hydrolytic stability and internal bond strength.